Re: [PATCH v4 1/5] mm,memory_hotplug: Allocate memmap from the added memory range

[David Hildenbrand]

On 18.03.21 13:03, Oscar Salvador wrote:
> On Thu, Mar 18, 2021 at 12:24:16PM +0100, David Hildenbrand wrote:
> > I don't follow. 2MB == 2MB. And if there would be difference then we would
> > be in the problem I brought up: vmemmap code allocating too much via the
> > altmap, which can be very bad because might be populating more vmemmap than
> > we actually need.
>
> Yes, I meant to say nr_vmemmap_pages won't match, or IOW, won't have the
> same meaning.
> The end result is the same.
>
> > vmemmap_size = 512 * 4KiB = 2 MiB.
> >
> > That calculation wasn't very useful (/ PAGE_SIZE * PAGE_SIZE)?
>
> Yeah, somewhat redundant.
>
> > >           unsigned long remaining_size = size - vmemmap_size;
> >
> > And here we could get something like
> >
> > remaining_size = 2 GiB - 2 MiB
>
> Yes, vmemmap_size would need to scale with nr_sections to be relative to
> size.
>
> Just wanted to bring it up, because somene might wonder
> "ok, why do we have altmap->nr_pfns = X, and here nr_vmemmap_pages
>   is Y"
>
> It was an effort to make it consistent, although I see it would bring
> more confusion other than anything, so disregard.

I am also unhappy that we have to replicate the same computation at two 
places, but I don't see an easy way to avoid that ... we have to trust 
on vmemmap code to do the right thing either way :(

--
Thanks,

David / dhildenb